1. Field of the Invention
The present invention relates to ceramic slurry compositions and methods for producing ceramic green sheets and multilayer ceramic electronic devices, and more particularly, relates to a ceramic slurry composition for use in production of ceramic electronic devices, such as multilayer ceramic capacitors and multilayer ceramic substrates, and to methods for producing a ceramic green sheet using the ceramic slurry composition and for producing a multilayer ceramic electronic device using the ceramic green sheets.
2. Description of the Related Art
Multilayer ceramic electronic devices, such as multilayer ceramic capacitors, and multilayer ceramic substrates, are generally produced by the steps of laminating ceramic green sheets, compressing the laminate and heating for sintering the ceramics and electrodes.
For example, when a multilayer ceramic capacitor as shown in FIG. 1 is produced in which internal electrodes 2 are formed in a ceramic element 1, and a pair of external electrodes 3a and 3b are formed at two side surfaces of the ceramic element 1 so as to be connected with the internal electrodes which alternately extend to one side surface and to the other side surface of the ceramic element 1, the method described below is generally used.
(1) Electrode-provided sheets 11 (see FIG. 2) are formed by disposing internal electrodes to be used as electrodes for a capacitor on the green sheets produced by the method mentioned above.
(2) A predetermined number of the electrode-provided sheets 11 are laminated as shown in FIG. 2, ceramic green sheets having no internal electrodes thereon (sheets used as outer layers) are disposed on the top and the bottom of the laminated sheets, and they are compressed, whereby a laminate (a compressed laminate) is formed in which ends of the internal electrodes 2 alternately extend to one side surface and to the other side surface of the laminate.
(3) The laminate is baked under predetermined conditions so as to be sintered, and an electroconductive paste is coated on two side surfaces of the baked laminate (the ceramic element 1) (see FIG. 1) and is baked, whereby the external electrodes 3a and 3b which are connected with the internal electrodes 2 are formed.
Accordingly, a multilayer ceramic capacitor as shown in FIG. 1 is produced.
Other multilayer ceramic electronic devices, such as multilayer ceramic substrates, are also produced by the step of laminating ceramic green sheets.
Ceramic green sheets for use in the production of multilayer ceramic electronic devices are generally formed by steps of preparing starting materials, such as a powdered ceramic, a dispersing medium (e.g., a solvent), a dispersing agent, a binder, and a plasticizer, so as to produce a predetermined composition; mixing and pulverizing the starting materials thus prepared by using a media-type mill, such as a bead mill, a ball mill, an attritor, a paint shaker and a sand mill, so as to form a ceramic slurry; molding the ceramic slurry into sheets having a predetermined thickness by methods such as the doctor blade method; and subsequently, drying the sheets thus formed. In this connection, the media-type mill mentioned above is an apparatus for dispersing a powdered ceramic between media by mixing and stirring the powdered ceramic with the media.
However, recently, miniaturization and improved performance have been required for various multilayer ceramic electronic devices, such as multilayer ceramic capacitors, as is the case with other electronic devices.
Accordingly, ceramic green sheets for use in the production of multilayer ceramic electronic devices must be thinner, and recently, use of extremely thin ceramic green sheets of 10 xcexcm or less thick is increasingly necessary.
When the extremely thin ceramic green sheets described above are produced, a ceramic slurry used for production of ceramic green sheets must be used in which the starting powdered ceramic is sufficiently dispersed. Hence, as a starting powdered ceramic, a fine powdered ceramic having an average particle diameter of 0.01 to 1 xcexcm must be used.
However, in a ceramic slurry used for the production of the ceramic green sheet as described above, in general, a dispersing agent is conventionally used which is a lower molecular compound of a binder in consideration of the compatibility with the binder.
That is, as a binder, polyvinyl butyral resins, cellulose resins, acrylic resins, vinyl acetate resins, polyvinyl alcohol resins and the like are often used, and hence, lower molecular compounds of the binder mentioned above are generally used as a dispersing agent.
In this connection, most of the resins used as binders, such as polyvinyl butyral resins, cellulose resins, acrylic resins, vinyl acetate resins and polyvinyl alcohol resins, are nonionic compounds, and as a result, the low molecular resins thereof used as dispersing agents are also nonionic compounds.
The nonionic dispersing agents mentioned above have low adsorbing rates on powdered ceramics, and hence, a fine powdered ceramic having particles of 1 xcexcm or less in diameter, which has strong cohesive force, cannot be rapidly and efficiently dispersed. Consequently, there are problems in that serious damage is done to the powdered ceramic and the productivity is decreased due to the longer time required in the dispersing step.
Accordingly, taking into consideration the problems described above, an object of the present invention is to provide a ceramic slurry composition having superior productivity in which a powdered ceramic can be efficiently dispersed without causing serious damage thereto, and to provide methods for producing ceramic green sheets using the ceramic slurry composition and for producing a ceramic electronic devices using the ceramic green sheets.
To these ends, a ceramic slurry composition of the present invention comprises a powdered ceramic, a dispersing agent, a binder and a solvent, wherein the dispersing agent is an anionic dispersing agent, and the content of the anionic dispersing agent is so that the total acid amount thereof corresponds to about 10 to 150% of the total base amount of the powdered ceramic.
As an anionic dispersing agent which is preferably used for the present invention, an anionic dispersing agent having intermolecular carboxyl groups, maleate groups, sulfonic groups, phosphate groups or the like is mentioned as an example. In addition, polycarboxylic compounds and polymaleate compounds containing no metal ions are mentioned as more preferable anionic dispersing agents.
The anionic dispersing agent is preferably added so that the total acid amount thereof corresponds to about 10 to 150% of the total base amount of the powdered ceramic. The reason for this is that when the dispersing agent is added so that the total acid amount thereof is less than about 10% of the total base amount of the powdered ceramic, satisfactory dispersing effects cannot be obtained, and on the other hand, when the total acid amount is more than about 150%, significant further improvement in the dispersing effects cannot be observed.
In this connection, the total acid amount of the anionic dispersing agent and the total base amount of the powdered ceramic can be determined by a titration method or the like.
In the present invention, a binder containing a plasticizer, and/or an anti-statistic agent may be used. In addition, a binder containing other additives may also be used.
In the present invention, the dispersing method for dispersing a powdered ceramic is not specifically limited. Various dispersing methods may be used, for example, a method of using a media-type mill, such as a bead mill, a ball mill, an attritor, a paint shaker and a sand mill; a method of kneading a powdered ceramic, a dispersing medium, a dispersing agent, a binder, a plasticizer and the like; and a method of using a three-roll mill. In this connection, the method of using a three-roll mill is a method for dispersing a powdered ceramic in a mixture thereof with a dispersing medium, a dispersing agent, a binder, a plasticizer, and the like. In the method described above, the mixture is passed through a small gap between a first roller and a second roller, which roll independently from each other and are adjacent to each other with the small gap therebetween, so as to be compressed and kneaded, and subsequently, the mixture is passed between the second roller and a third roller, which rolls and is adjacent to the second roller with a smaller gap therebetween than the gap between the first and the second rollers, so as to be further compressed and kneaded.
In addition, when the ceramic slurry composition of the present invention is formed, the sequence of addition of dispersing agent and binder is not specifically limited. However, it is generally preferable that powdered ceramic, dispersing agent, and solvent be mixed and dispersed so that the dispersing agent is adsorbed on the powdered ceramic beforehand; a binder is then added to the mixture thus formed, and subsequently, mixing and dispersing is performed again.
In the ceramic slurry composition according to the present invention, the average particle diameter of the powdered ceramic is preferably about 0.01 to 1 xcexcm.
According to the present invention, a ceramic slurry composition provided with superior dispersibility of a powdered ceramic having diameters of about 0.01 to 1 xcexcm (the average particle diameter measured by an electron microscope), which is generally difficult to disperse by a conventional dispersing method, can be obtained. Hence, the present invention is particularly significant.
In addition, the present invention can be used when the particle diameter of a powdered ceramic is out of the range of about 0.01 to 1 xcexcm.
A method for producing a ceramic green sheet of the present invention comprises the step of molding the ceramic slurry composition described above into a sheet on a predetermined substrate so as to form the green sheet.
Since the powdered ceramic is sufficiently dispersed in the ceramic slurry composition according to the present invention described above, the thin ceramic green sheets having high quality can be reliably produced by molding the ceramic slurry composition into sheets. That is, a ceramic green sheet preferably used for production of multilayer ceramic electronic devices can be produced, in which the ceramic green sheet has superior smooth surfaces, a high density and a high tensile strength, and in which resins, such as a binder and a plasticizer, are uniformly distributed therein. Furthermore, when a multilayer ceramic electronic device is produced by using the ceramic green sheets described above, a highly reliable multilayer ceramic electronic device having desired characteristics and high quality can be obtained.
In the method for producing the ceramic green sheet according to the present invention, the thickness of the ceramic green sheet is preferably about 0.1 to 10 xcexcm.
According to the present invention, even when the ceramic green sheet is formed to be thin from about 0.1 to 10 xcexcm, a ceramic green sheet having high quality can be reliably produced, and hence, ceramic green sheets preferably used for production of multilayer ceramic electronic devices can be obtained.
A method for producing a multilayer ceramic electronic device, according to the present invention, comprises a step of laminating the ceramic green sheets produced by the method for producing the ceramic green sheets described above together with internal electrodes composed of a base metal, a step of cutting the laminated ceramic green sheets, a step of baking the laminated ceramic green sheets and a step of forming external electrodes.
A highly reliable multilayer ceramic electronic device having desired characteristics and high quality can be formed by the steps of producing ceramic green sheets using the ceramic slurry formed by the method according to the present invention described above, laminating the ceramic green sheets together with the internal electrodes composed of a base metal, cutting, baking, and forming the external electrodes.
A method for producing multilayer ceramic electronic devices of the present invention, comprises steps of laminating the ceramic green sheets, which are produced by the method for producing the green sheets described above, together with the internal electrodes composed of a base metal; cutting; baking; and forming the external electrodes. Since the ceramic green sheets having a higher density and superior smooth surfaces are used, the rate of occurrence of short-circuiting can be decreased, and hence, the reliability thereof can be improved. In addition, since serious damage is not done to a powdered ceramic, the reproducibility of the target characteristics can be improved.